Critical cluster composition from homogeneous nucleation data: application to water in carbon dioxide–nitrogen carrier gases

Knowledge on critical cluster composition is important for improving the nucleation theory. Thus, homogeneous water nucleation experiments previously carried out in nitrogen and 0%, 5%, 15% and 25% of carbon dioxide ( Campagna et al. 2020a, 2021) are analyzed. The tests were conducted at 240 K and 0.1 MPa, 1 MPa and 2 MPa. The observed nucleation rates are strongly dependent on supersaturation, pressure, temperature and mixture composition. These experimentally found dependencies can be used to derive the composition of critical clusters by means of the nucleation theorem. In this way, a macroscopic quantity, nucleation rate, reveals properties of critical clusters consisting of a few tens of molecules. Two novel methods are presented for the detailed application of the nucleation theorem. The first method extends to mixtures of $$\,\,\,\,\,\,\,N>2\,\,\,\,\,\,$$ N > 2 components the approach used in literature for two components. The second method not only applies to $$N>2$$ N > 2 mixtures in a more straightforward manner, but it can also be used for unary as well as for binary and multi-component nucleation cases. To the best of our knowledge, for the first time the critical cluster composition is computed for high pressure nucleation data of a vapor (here water) in mixtures of two carrier gases (here carbon dioxide–nitrogen). After a proper parameterization of the nucleation rate data, both methods consistently lead to the same critical nuclei compositions within the experimental uncertainty. Increasing pressure and carbon dioxide molar fraction at fixed supersaturation leads to a decrease in the water content of the critical cluster, while the adsorbed number of nitrogen and carbon dioxide molecules increases. As a consequence, the surface tension decreases. This outcome explains the observed increase in the nucleation rate with increasing pressure and carbon dioxide molar fraction at constant supersaturation. Graphic abstract

Nucleation of Ga droplets self-assembly on GaAs(111)A substrates

We investigated the nucleation of Ga droplets on singular GaAs(111)A substrates in the view of their use as the seeds for the self-assembled droplet epitaxial quantum dots. A small critical cluster size of 1–2 atoms characterizes the droplet nucleation. Low values of the Hopkins-Skellam index (as low as 0.35) demonstrate a high degree of a spatial order of the droplet ensemble. Around $$350\,^{\circ }\hbox {C}$$ 350 ∘ C the droplet size distribution becomes bimodal. We attribute this observation to the interplay between the local environment and the limitation to the adatom surface diffusion introduced by the Ehrlich–Schwöbel barrier at the terrace edges.

Nucleation of Ga Droplets Self-Assembly on GaAs(111)A Substrates

We investigated the nucleation of Ga droplets on exact GaAs(111)A substrates in the view of their use as the seeds for the self-assembly droplet epitaxial quantum dots. A small critical cluster size of 1-2 atoms characterizes the droplet nucleation. Low values of the Hopkins-Skellam index (as low as 0.35) demonstrate a high degree of a spatial order of the droplet ensemble. Around 350 °C the droplet size distribution becomes bimodal. We attribute this observation to the interplay between the local environment and the limitation to the adatom surface diffusion introduced by the Ehrlich-Schwöbel barrier at the terrace edges.

Bacteria as Cloud Condensation Nuclei (CCN) in the Atmosphere

Bacteria activation and cloud condensation nuclei (CCN) formation have been studied in the atmosphere using the classical theory of heterogeneous nucleation. Simulations were performed for the binary system of sulfuric acid/water using laboratory-determined contact angles. Realistic model simulations were performed at different atmospheric heights for a set of 140 different bacteria. Model simulations showed that bacteria activation is a potentially favorable process in the atmosphere which may be enhanced at lower temperatures. CCN formation from bacteria nuclei is dependent on ambient atmospheric conditions (temperature, relative humidity), bacteria size, and sulfuric acid concentration. Furthermore, a critical parameter for the determination of bacteria activation is the value of the intermolecular potential between the bacteria’s surface and the critical cluster formed at their surface. In the classical nucleation theory, this is parameterized with the contact angle between substrate and critical cluster. Therefore, the dataset of laboratory values for the contact angle of water on different bacteria substrates needs to be enriched for realistic simulations of bacteria activation in the atmosphere.

Heterogeneous Nucleation in Solutions on Rough Solid Surfaces: Generalized Gibbs Approach

Heterogeneous nucleation of new phase clusters on a rough solid surface is studied. The ambient phase is considered to be a regular supersaturated solution. In contrast to existing studies of the same problem, the possible difference between the state parameters of the critical cluster and the corresponding parameters of a newly formed macroscopic phase is accounted for. This account is performed within the framework of the generalized Gibbs approach. Surface imperfections are chosen in the form of cones. The model allows us to simplify the analysis but also to obtain the basic results concerning the defect influence on the nucleation process. It is shown that the catalytic activity factor for nucleation of the cone depends both on the cone angle and the supersaturation in the solution determining the state parameters of the critical clusters. Both factors considerably affect the work of critical cluster formation. In addition, they may even lead to a shift of the spinodal curve. In particular, in the case of good wettability (macroscopic contact angle is less than 90 ∘ ) the presence of surface imperfections results in a significant shifting of the spinodal towards lower values of the supersaturation as compared with heterogeneous nucleation on a planar solid surface. With the decrease of the cone pore angle, the heterogeneous spinodal is located nearer to the binodal, and the metastability range is narrowed, increasing the range of states where the solution is thermodynamically unstable.

Nucleation of antagonistic organisms and cellular competitions on curved, inflating substrates

We consider the dynamics of spatially-distributed, diffusing populations of organisms with antagonistic interactions. These interactions are found on many length scales, ranging from kilometer-scale animal range dynamics with selection against hybrids to micron-scale interactions between poison-secreting microbial populations. We find that the dynamical line tension at the interface between antagonistic organisms suppresses survival probabilities of small clonal clusters: the line tension introduces a critical cluster size that an organism with a selective advantage must achieve before deterministically spreading through the population. We calculate the survival probability as a function of selective advantage δ and antagonistic interaction strength σ. Unlike a simple Darwinian selective advantage, the survival probability depends strongly on the spatial diffusion constant Ds of the strains when σ > 0, with suppressed survival when both species are more motile. Finally, we study the survival probability of a single mutant cell at the frontier of a growing spherical cluster of cells, such as the surface of an avascular spherical tumor. Both the inflation and curvature of the frontier significantly enhance the survival probability by changing the critical size of the nucleating cell cluster.

Experimental study of H₂SO₄ aerosol nucleation at high ionization levels

One hundred and ten direct measurements of aerosol nucleation rate at high ionization levels were performed in an 8 m3 reaction chamber. Neutral and ion-induced particle formation from sulfuric acid (H2SO4) was studied as a function of ionization and H2SO4 concentration. Other species that could have participated in the nucleation, such as NH3 or organic compounds, were not measured but assumed constant, and the concentration was estimated based on the parameterization by Gordon et al. (2017). Our parameter space is thus [H2SO4] =4×106-3×107 cm−3, [NH3+ org] = 2.2 ppb, T=295 K, RH = 38 %, and ion concentrations of 1700–19 000 cm−3. The ion concentrations, which correspond to levels caused by a nearby supernova, were achieved with gamma ray sources. Nucleation rates were directly measured with a particle size magnifier (PSM Airmodus A10) at a size close to critical cluster size (mobility diameter of ∼ 1.4 nm) and formation rates at a mobility diameter of ∼ 4 nm were measured with a CPC (TSI model 3775). The measurements show that nucleation increases by around an order of magnitude when the ionization increases from background to supernova levels under fixed gas conditions. The results expand the parameterization presented in Dunne et al. (2016) and Gordon et al. (2017) (for [NH3+org] = 2.2 ppb and T=295 K) to lower sulfuric acid concentrations and higher ion concentrations. The results make it possible to expand the parameterization presented in Dunne et al. (2016) and Gordon et al. (2017) to higher ionization levels.

Experimental study of H₂SO₄ aerosol nucleation at high ionization levels

One hundred and ten direct measurements of aerosol nucleation rate at high ionization levels were performed in an 8 m3 reaction chamber. Neutral and ion-induced particle formation from sulphuric acid (H2SO4) as a function of ionization and H2SO4 concentration was studied. Other species that could participate in the nucleation were not measured. The measurements extend the parameter space of measurements described by (Dunne, 2016) (at T = 295 K and RH = 38 %) by expanding to lower H2SO4 concentrations (4x106–3x107 cm−3) and higher ion concentrations (1700–19000 cm−3). The ion concentrations, which correspond to levels caused by a nearby supernova, were achieved with gamma ray sources. Nucleation rates were directly measured with a particle size magnifier (PSM Airmodus A10) at a size close to critical cluster size (mobility diameter of ~ 1.4 nm) and formation rates at mobility diameter of ~ 4 nm were measured with a CPC (TSI model 3775). The measurements show that nucleation increases by around a factor of five when the ionization increases from background to supernova levels under fixed gas conditions. The results expand the parametrization from (Dunne, 2016) to lower sulphuric acid concentrations and higher ion concentrations.